Add support for requiring/enforcing client TLS certificate on front end interface

[alegacy]

To support operating in environments that require/enforce client TLS certificates it would be helpful to support configurability on the front end HTTPS interface to set the ClientAuthType to RequireAndVerifyClientCert.

This would required new command line argument similar to:

	flagset.BoolVar(&o.TLS.RequiredClientCert, "tls-require-client-cert", false, "When set to true incoming clients must supply a valid client certificate.  If set then client-ca-file must also be set.")